KMS on Server 2003 and 2008 should each behave the same way. In fact, if you want to use both Server 2003 and Server 2008 to host KMS, the clients can move between the hosts transparently to the end user. The "rules" for number of activations to start activating servers (5) and clients (25) are the same across both platforms, and you should use the same key to activate KMS regardless of whether it is running on 2003 or 2008.

You can apply this as an update to KMS 1.0 on Server 2003 (you do not have to uninstall KMS 1.0 first, according to the documentation).

The same MOM management pack that applied to KMS 1.0 applies to KMS 1.1.

As with KMS 1.0 for Server 2003, there is no SLUI and you still need to use a 2003 key and activate the OS on the machine where you are installing the Key Management Service, as a contrast to 2008 where once the key has been entered to make the machine a KMS and the KMS has been activated, the machine hosting KMS does not require its own key and activation.